The present invention relates generally to the field of x-ray imaging, and more particularly to 4-dimensional (4D) digital tomosynthesis and its applications in radiation therapy.
Digital tomosynthesis (DTS) reconstructs structures existing within an imaged object from a set of projection radiographs. In medical applications, these structures include, for example, anatomical structures such as organs, blood vessels, and bones. In computed tomography (CT), both an x-ray source and an x-ray detector move on a circular trajectory around a common axis, and a very high number of projection radiographs (or images) is acquired. In contrast, in tomosynthesis, relatively few radiographs are acquired for varying x-ray source positions. Tomosynthesis, then, is a system and method that acquires a plurality of projection radiographs, which is not enough for exact computed tomography. In tomosynthesis, typically the x-ray source assumes positions that are essentially on one side of the object, while the detector (or film) assumes positions on the opposite side of the object.
DTS is a method of reconstructing cross sections of a 3D body from its 2D radiographic projections, which is a much faster method than the CT approach for obtaining cross sections. In CT, projections must be acquired from at least 180 degrees plus the fan angle around the object to produce an exact reconstruction of the object. DTS, however, exploits projections from limited angles to reconstruct cross sections of the object. Although the reconstruction is less precise, and the plane of reconstruction is limited to one orientation only, it has the benefit of using a smaller number of projections, i.e. scan angle. This translates into faster data acquisition and provides the advantage of being able to reconstruct objects where space and size limitations prevent one from acquiring projections from all angles. In some clinical situations, exact reconstruction is not necessary, making a fast DTS ideal.
A DTS system includes an x-ray source and a digital detector which are connected to each other by an appropriate mechanical structure. In conventional 3-D DTS, a number of 2-dimensional projection radiographs of a stationary imaged object is acquired at different positions of the x-ray source relative to the imaged object, and from the data sets corresponding to the 2-dimensional projection radiographs, cross sections of the imaged object are reconstructed.
Cone Beam Computed Tomography (CBCT) is expected to play a significant role in radiation therapy. However, due to safety concerns the maximum speed of gantry rotation (e.g., for a Linac machine) is currently limited to ˜1.0 rpm (rotation per minute). As a result, data acquisition for CBCT is typically long (on the order of one minute). To ensure image quality, the subject must be motionless during the acquisition. However, there are certain physiological motions such as breathing that cannot be stopped for the duration of acquisition. Therefore, motion artifacts will always plague slow CBCT data acquisition.
It is desired to reconstruct 3D cross sections of time-varying objects at different time instances of the physiological cycle. For example, it would be beneficial to show cross sections of a patient's lungs during the entire breathing cycle. That would allow clinicians to pinpoint the location of a time varying lung tumor at every phase of the breathing cycle. Thus, they would be able to locate and delineate the tumor at exhalation.
Consequently, it would be desirable to provide a new digital tomosynthesis system and method in 4D radiation therapy where the time component is integrated into the three-dimensional (3D) radiation therapy process to deliver dose in view of target motion.